1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device and, more specifically, to an annealing process in a method of fabricating a semiconductor device.
2. Description of the Related Art
When fabricating a semiconductor device, a plurality of semiconductor elements are formed on a semiconductor substrate, and the semiconductor substrate carrying the semiconductor elements is subjected to a high-temperature heat treatment to isolate the semiconductor elements from each other or to connect the semiconductor elements. The semiconductor substrate is subjected to an ion implanting process to form lightly doped drain (LDD) structures and source-drain regions in the surface thereof. The semiconductor substrate processed by the ion implanting process needs to be subjected to an activating annealing process to restore the crystallinity of the semiconductor substrate and to electrically activate acceptor ions and doner ions implanted in the surface of the semiconductor substrate.
Silicides, such as the silicides of metals having high melting points such as W, Mo and Ti, Pt and Pd, are used because the electrical resistance of silicides is lower than that of polycrystalline silicon. The depth of penetration of a laser beam into a silicide film is on the order of several tens nanometers. Furthermore, silicides need to be heated to a high temperature by a high-temperature heat-treating process to reduce contact resistance. A furnace annealing method and a rapid thermal annealing method have been applied to the activating annealing process and the high-temperature heat-treating process.
The component semiconductor elements of a semiconductor device of a high degree of integration are miniaturized, requiring shallow junctions in the source-drain regions. When the semiconductor substrate is annealed for activating annealing by the furnace annealing method or the rapid thermal annealing method, the depth of the diffused layer is increased and, consequently, it is impossible to form shallow junctions in the source-drain regions to miniaturize the component semiconductor elements to form the semiconductor device in a high degree of integration. An activating annealing method employing a pulse laser beam is one of the shallow junction forming methods.
The depth of a layer which can be annealed by a pulse laser beam is 100 nm or less because the energy of the pulse laser beam is absorbed by the very thin surface layer of a thickness of about 20 nm of the semiconductor substrate. Therefore, the annealing method employing a pulse laser beam is suitable for the activating annealing for forming LDD structures or source-drain regions. However, it is difficult to reduce the resistance of the entire area of a portion of the silicide layer extending over the gate electrode simultaneously with the activating annealing of the LDD structures and source-drain regions because a silicide layer formed, for example, over a gate electrode has a thickness of 100 nm or above.
Such a difficulty may be overcome by using a laser having a higher power capacity. However, a pulse laser beam having increased energy causes the acceptor ions and doner ions to diffuse into the depth of the source-drain regions to form deep junctions in the LDD structures or source-drain regions. If the pulse laser beam has low energy, only the surface layer of a very small thickness of the semiconductor substrate is melted and the surface of the semiconductor substrate becomes flat immediately after the removal of the pulse laser beam. On the other hand, when a pulse laser beam of high energy is used, the surface layer of a considerably large thickness of the semiconductor substrate is melted to deteriorate the flatness and smoothness of the surface of the semiconductor substrate. Furthermore, it is difficult to process a plurality of regions differing from each other in thickness or depth simultaneously with such a pulse laser beam having high energy.
It is very difficult to apply a laser beam annealing process to fabricating a semiconductor device having a plurality of laminated layers because the pulse laser beam is intercepted by the upper layers and unable to reach the lower layers.